Johnson & Johnson said its Covid vaccine is effective against the highly contagious Delta variant, which complements the growing body of evidence that the most widely used Covid vaccines offer protection against its most dangerous variants.

Even eight months after the vaccination, the single-shot J. & J. The vaccine is proving highly effective against Delta, the company reported Thursday, a reassuring finding for the 11 million Americans who have received the vaccine and countries around the world that are betting on receiving the vaccine. In the United States, the variant first identified in India is now an estimated one in four new cases, and the CDC has listed it in 23 states.

Johnson & Johnson said its vaccine had a small decrease in effectiveness against the Delta variant compared to its effectiveness against the original virus and a larger decrease against the beta variant first identified in South Africa. This is the same pattern as the Pfizer-BioNTech and Moderna mRNA vaccines.

The intense discourse on Deltas threat has made some people who have been vaccinated fearful of whether they are protected. The worldwide spread of the variant has led to new restrictions from Ireland to Malaysia.

The lack of clarity about the effectiveness of the Johnson & Johnson vaccine against Delta had frustrated. And reports of a cluster of cases among players on the Yankees baseball team who received the Johnson & Johnson shot, even though all of them were asymptomatic or mild, did not help allay the fears.

Studies have shown that the Delta and Beta variants slightly decrease the effectiveness of the Pfizer and Moderna vaccines. For Pfizer, studies show that two doses offer 88 percent protection against the Delta variant, just below the 93 percent protection against alpha. The Moderna vaccine has performed similarly to Pfizer’s in previous studies.

Johnson & Johnson has collected less data on the vaccines than its counterparts, and the study published Thursday was small and has not yet been published in a scientific journal. The update of the effectiveness of the Johnson & Johnson vaccine has been slow as it was later introduced in the United States as the Pfizer and Moderna vaccines. The vaccine offered about 72 percent protection against early versions of the virus.